The major structural characteristic of avian gastrulation is the primitive streak, which becomes the blastopore lips of amniotic embryos. Migration through the primitive streak is controlled by Fgf8. What would happen if the Fgf8 protein, which repels migrating cells away from the streak, is over expressed in the primitive streak?
The yolk sac will be deformed.
Avian Gastrulation and Primitive Streak
Avian gastrulation is a fundamental process in bird development where the three germ layers (ectoderm, mesoderm, and endoderm) are established, and the basic body plan is laid out. A key structure in avian gastrulation is the primitive streak, which is functionally equivalent to the blastopore in other amniotic embryos.
Cells migrate through the primitive streak to ingress into the embryo, forming the mesoderm and endoderm. This migration is a tightly regulated process involving various signaling molecules.
Role of Fgf8 in Cell Migration
The question mentions that the protein Fgf8 plays a role in controlling migration through the primitive streak and that it repels migrating cells away from the streak. This implies that Fgf8 sets up a repulsive gradient or signal that influences the direction or behavior of cells as they move through or near the streak.
Impact of Fgf8 Overexpression
If Fgf8 protein is overexpressed in the primitive streak, the repulsive signal is amplified or extended further. This would disrupt the normal pattern of cell migration. Cells that are supposed to ingress through the streak and move to specific locations within the forming embryo or extraembryonic membranes would be excessively repelled or misdirected.
Normal gastrulation is crucial for the proper formation of all embryonic and extraembryonic tissues, including the yolk sac. The mesodermal layer of the yolk sac is formed by cells that migrate from the primitive streak.
Overexpression of Fgf8, a repulsive signal emanating from the streak, would severely impair the ability of cells, including those destined for the extraembryonic mesoderm like the yolk sac mesoderm, to migrate correctly. This disruption in the formation and organization of the yolk sac mesodermal layer would lead to structural defects.
Evaluating the Options
- Option 1: The yolk sac will be deformed. This aligns with the understanding that misregulated cell migration during gastrulation due to overexpression of a repulsive factor like Fgf8, particularly affecting cells contributing to extraembryonic structures like the yolk sac, would likely result in structural abnormalities or deformation of the yolk sac.
- Option 2: Wnt signalling will be activated and orientation of the primitive streak will change. While Wnt signaling is involved in primitive streak formation and maintenance, the primary stated function of Fgf8 in this context is cell repulsion during migration. Overexpression of Fgf8 is more directly linked to migration issues rather than altering the orientation of the streak itself.
- Option 3: Cells of the streak will not form the paraxial mesoderm. Cells migrating through the streak do contribute to mesoderm, including paraxial mesoderm. Disruption by Fgf8 overexpression would likely impair the proper formation or location of mesoderm, but stating they "will not form" it at all might be too absolute a consequence. The issue is more about where they end up or if migration fails completely.
- Option 4: Cells generate mesodermal portions of the embryo. This describes the normal outcome of gastrulation, not a consequence of Fgf8 overexpression. The overexpression would disrupt this normal process.
Therefore, the most likely consequence of overexpressing a repulsive factor like Fgf8 in the primitive streak, given its role in controlling cell migration and the critical dependence of structures like the yolk sac on proper cell movement from the streak, is a deformation of the yolk sac.
Important Questions from Morphogenesis and organogenesis in animals
The individuals considered in this question are having two haploid sets of autosomes and no Y- chromosome. The X∶A ratio of the individuals, the type of organisms chosen, their primary sex and number of Barr bodies expected in their cells are shown in the table below:
X∶A ratio Organism Primary Sex Number of Barr bodies i. 0.5
ii. 2
A. Human
B. Drosophila
I. Male
II. Female
III. Metafemale
a. Zero
b. One
c. Two
d. Three
Select the option below with all correct matches:
Several marine organism release their gametes into the environment, where sperm attraction and subsequent events lead to successful fertilization. With reference to sea urchins, which one of the following statements is NOT true?
The following statements regarding the generation of dorsal/ventral axis in Drosophila was made:
A. Gurken protein moves along with the oocyte nucleus and signals follicle cells to adopt the ventral fate.
B. Maternal deficiencies of either the gurken or torpedo gene cause ventralization of the embryo.
C. Gurken is active only in the oocyte and Torpedo is active only in the somatic follicle cells.
D. The Pipe protein is made in the dorsal follicle cells.
E. The highest concentration of Dorsal is in the dorsal cell nuclei, which becomes the mesoderm.
Which one of the following combination of the above statements is true?
During wing development in chick, if Apical Ectodermal Ridge (AER) is removed, the limb development ceases, on the other hand placing leg mesenchyme directly beneath the wing AER, distal hindlimb structures develop at the end of the wing, and if limb mesenchyme is replaced by non-limb mesenchyme beneath AER, the AER regresses. This may demonstrate that:
A. the limb mesenchyme cells induce and sustain AER.
B. the mesenchyme cells specify the type: wing or limb.
C. the AER is responsible for specifying the type: wing or limb.
D. the AER is responsible for sustained outgrowth and development of the limb.
E. the AER does not specify the type: wing or limb.
Which combination of above statements is demonstrated by the experiment?
The following statements are made regarding the amphibian early-embryonic development:
A. The Nieuwkoop center cells are mesodermal in origin.
B. Chordin, Noggin and Goosecoid are secreted by the Organizer.
C. The default fate of the ectoderm is to become neural tissue.
D. BMP levels are high in the presumptive dorsal mesoderm.
Which one of the following options represents the combination of all correct statements?
